It's really too difficult to actually give any definitive account of who or what MT is given he's been in it so little. But I agree with you that he's undoubtedly a character of huge significance - he has his own chapter which comes at the start of an arc, RB seem to be fascinated with him. He's definitely going to be some sort of key to their arc.

My own hunch is that the Beast Titan is a creature that occupies some kind of near-mythic status in RBA's village. That would explain RB's reaction upon seeing it - they had been told about this creature all their lives and finally it's just there.

I don't think he was the first titan - just because that would seem too much of a giveaway at this point in the plot - I think, though, that he definitely has some direct involvement with the first "batch" of Titans that probably existed around the time the progenitor titan created the walls. The one thing we seem to know about him is that his information about humanity comes from second hand sources. It's often said that he didn't think he spoke the same language as humans. Maybe it's an error in translation but what I got was that when Mike froze up, he says "I thought we spoke the same language" - implying he'd been informed by a third party that he would be able to speak to humans, and was surprised that this information appeared to be incorrect.

The fact that he had to infer that Mike was terrified of him suggests that he has had little if any contact with humanity whatsoever - if he'd have met humans before he would be used to them running scared, surely? If RBA's mission was in part to find the Beast Titan is it not possible that the purpose of the Colossal and Armored Titan was to actually lure the beast titan and Annie's role was to capture the co-ordinate? When you consider that the former two have a limited purpose (breaking down the walls) and Annie seems to be more agile and subtle in general, it's not too ridiculous to consider - they all seem like their purpose was far more than to simply incur a titan invasion.

All of this is complete speculation, because like I say the chapters in which he features leave very little clues, but it seems obvious to me that he has some kind of huge significance to the plot that has yet to be realised. It's worth mentioning that MT hasn't featured in Rod Reiss' ramblings nor has he appeared in Eren's recollections - which I know isn't a slam-dunk, but it makes me think his importance isn't world-historic, like MT being the first king or something, but that he is instead a product of something which came after the creation of the walls.

Anyway, short answer is "who knows" interested if anyone else has any ideas.